Indium-lll-labeled autologous platelets have been demonstrated to localize in a time-dependent and reproducible pattern on polyethylene angiographic catheters in the dog carotid artery, permitting quantitative evaluation of in vivo thrombus formation. These studies will be extended to evaluate in vivo thrombus formation related to angiographic jet trauma, acute myocardial infarction, and experimental pulmonary embolism in the dog. In addition, the In-lll-platelet technique will be extended to several clinical patient populations, including patients with extracranial cerebrovascular atherosclerosis, acute thrombophlebitis with or without pulmonary embolism, acute myocardial infarction, and post-surgical cardiovascular patients who have received coronary artery bypass grafts, arterial prostheses, or cardiac valve grafts. This quantitative technique should permit evaluation of specific antithrombotic therapy (heparin, aspirin, persantin, etc.) both in the experimental canine model as well as in specific clinical entities in man.